Oversampling A/D converter

Abstract

An oversampling A/D converter includes a first filter including a first resistive element, a first capacitive element, a second resistive element, an operational amplifier, and a second capacitive element; a second filter receiving an output of the first filter; a third filter including a third resistive element, a third capacitive element, and a fourth resistive element; a quantizer receiving an output of the third filter and generating a digital signal; and a D/A converter converting the digital signal to an analog current signal. The D/A converter inputs the generated analog current signal to an inverting input terminal of the operational amplifier.

What is claimed is: 
     
       1. An oversampling A/D converter configured to perform digital conversion of an input signal, the oversampling A/D converter comprising:
 a first filter configured to receive the input signal and a first analog signal and to output a first output signal, 
 a continuous-time filter configured to receive the first output signal and to output a second output signal, 
 a second filter configured to receive the input signal and the second output signal and to output a third output signal, 
 a quantizer configured to receive the third output signal and to output a digital signal, and 
 a first D/A converter configured to receive the digital signal and to output the first analog signal, 
 wherein the second filter sums the input signal and the first output signal and has no active element between the input signal and the second output signal. 
 
     
     
       2. The oversampling A/D converter of  claim 1 , wherein the first filter comprises an operational amplifier coupled to the first output signal. 
     
     
       3. The oversampling A/D converter of  claim 2 , wherein the operational amplifier is coupled to the first analog signal. 
     
     
       4. The oversampling A/D converter of  claim 1 , further comprising:
 a second D/A converter configured to receive the digital signal and to output a second analog signal, 
 wherein the second filter receives the second analog signal. 
 
     
     
       5. The oversampling A/D converter of  claim 1 , wherein the second filter has only a resistive element between the input signal and the second output signal. 
     
     
       6. The oversampling A/D converter of  claim 1 , wherein the oversampling A/D converter has a differential system. 
     
     
       7. The oversampling A/D converter of  claim 6 , wherein the input signal, the first output signal, the second output signal, the third output signal and the first analog signal each comprise differential signals.